Active pixel sensors often are arranged in arrays of pixels. Each pixel represents one part of the image. The pixels are read out in groups, e.g., one column at a time. It is desirable that these columns be read out at high speed. It is also desirable that the active pixel sensors have as much resolution as possible. This requires a larger array--that means a larger number of columns and a larger number of elements in the columns. Each device in each column is in parallel with a switch controlling when the device is turned on. Therefore, adding more devices puts more devices in parallel.
An exemplary device is shown in FIG. 1. Each column 100 may have a large number of devices in the column. The source follower 102 drives the entire column, and hence drives the entire analog bus capacitance of the column. A column select switch 104 selects the column. A larger column requires a more powerful column select switch and a more powerful column source follower in order to drive the entire analog bus capacitance. The more powerful switch typically requires a larger transistor, with a longer channel. This itself makes a higher total bus capacitance, and makes it even harder to drive the bus. The stray capacitance 112 of the bus therefore increases. Therefore, the transistor size increases, causes the capacitance to increase, and therefore to require an even larger transistor to drive the capacitance. Eventually the system reaches a point of diminishing returns.